This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Basic and Applied Ecology 12 (2011) 1–9 Plant invasions in temperate forests: Resistance or ephemeral phenomenon? Franz Essla,∗ , Norbert Milasowszkyb , Thomas Dirnböcka a b Environment Agency Austria, Spittelauer Lände 5, A-1090 Vienna, Austria Institut für angewandte Biologie und Umweltbildung (IFABU), Argentinierstraße 54/21, A-1040 Vienna, Austria Received 23 March 2010; accepted 13 October 2010 Abstract Invasion patterns in Europe are uneven across habitats. In particular, temperate zonal forests are relatively little affected by invasions. However, long generation periods of forest species and the rarity of disturbance events may lead to long time lags, and so, invasion resistance of temperate forests may have been overestimated. Based on the inventory of the alien vascular flora of 15 study sites in East Austrian lowland forest patches adjacent to villages, we analysed diversity and temporal trends of their alien flora and the influence of within-habitat diversity, occurrence of forest areas dominated by alien tree species and adjacent land use diversity on the level of invasion. In total, we recorded 119 alien (neophyte) species. Species introduced by horticulture are of overriding importance (86% of all aliens). Using the year of first record in Austria as a proxy for local residence time, numbers of aliens have increased linearly by five species per decade since 1800. A GLM including the study site size and ratio of forest areas dominated by alien vs. native tree species explains 64% of the variance of the number of alien species. Size was more important, but the ratio of forest areas dominated by alien trees (Ailanthus altissima, Robinia pseudacacia) significantly increased alien species number. Our results show, that invasion of alien trees with strong impacts on ecosystem properties facilitates invasion in Central European forests, leading to “invasional meltdown”. Further, high levels of propagule pressure, created by adjacent settlements and gardens, may foster invasions. Currently observed low levels of invasion in Central European forests situated in greater distance to settlements may turn out to be an ephemeral phenomenon. Spread of alien forest plants on the landscape level may ultimately, although possibly only over long time periods, lead to increased levels of invasion. Zusammenfassung Neophyten besiedeln unterschiedliche Lebensräume Europas in unterschiedlichem Ausmaß. Im Besonderen gelten temperate zonale Wälder als nur wenig von biologischen Invasionen betroffen. Allerdings können die lange Generationszeit von Waldarten und die Seltenheit von Störungserereignissen zu besonders langen Verzögerungen von Invasionen führen, sodass die Invasionsresistenz temperater Wälder möglicherweise überschätzt wird. Auf Basis eines Inventars der Neophytenflora von 15 an Siedlungen angrenzenden Waldflächen im östlichen Tiefland Österreichs untersuchten wir Diversität und zeitliche Trends der Neophytenflora sowie die Bedeutung der Habitatvielfalt, des Vorkommens von durch nichtheimische Baumarten dominierten Waldflächen und angrenzender Landnutzung. ∗ Corresponding author. Tel.: +43 1 31303 3323; fax: +43 1 31304 3700. E-mail address: franz.essl@umweltbundesamt.at (F. Essl). 1439-1791/$ – see front matter © 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.baae.2010.10.003 Author's personal copy 2 F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9 Insgesamt fanden wir 119 Neophytentaxa. Aus Gartenkultur verwilderte Arten dominierten (86% aller Neophyten). Auf Basis des Erstnachweises der Arten in Österreich als Maßzahl für lokale Anwesenheitszeit wuchs die Neophytenflora mit einer linearen Zunahme von 5 Arten pro Dekade seit 1800 an. In den letzten Jahrzehnten haben Sträucher, aus Asien stammende Arten, in Gartenkultur entstandene Sippen (Anökophyten) und als Gartenpflanzen eingeführte Arten überproportional zugenommen. Ein GLM mit Flächengröße und dem Anteil von durch nichtheimische Baumarten dominierten Waldflächen erklärt 64% der Varianz der Neophytenzahl. Flächengröße war wichtiger, aber der Anteil von durch nichtheimische Baumarten (Ailanthus altissima, Robinia pseudacacia) dominierten Waldflächen erhöhte ebenfalls die Neophytenzahl signifikant. Unsere Ergebnisse zeigen, dass das Eindringen nichtheimischer Baumarten mit starken ökosystemaren Auswirkungen die Ausbreitung anderer Neophyten in mitteleuropäischen Wäldern begünstigt und zu einem “invasional meltdown” führt. Angrenzende Siedlungen und Gärten scheinen wegen des hohen Samendrucks Invasionen zu begünstigen. Die derzeit beobachtete geringe Ausbreitung von Neophyten in mitteleuropäischen Wäldern in größerem Abstand zu Siedlungen mag sich daher zukünftig als ein temporäres Phänomen herausstellen. Die Ausbreitung von Neophyten auf der Landschaftsebene mag letztlich, wenngleich vermutlich über längere Zeiträume, zu einem höheren Invasionsniveau führen. © 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. Keywords: Central Europe; Habitat diversity; Horticulture; Invasional meltdown; Life form; Pathway; Propagule pressure; Residence time; Temporal change; Vascular plants Introduction The rate of biological invasions is rapidly increasing worldwide (Lambdon et al. 2008; Hulme, Pyšek, Nentwig, & Vilá 2009). Invasive species (transformer species sensu Richardson et al. 2000) are causing large detrimental effects on both biodiversity and human well-being (Vilà et al. 2009). Recent research has demonstrated that, besides climate matching, human activities are an important determinant of the biogeography of invasions (Taylor & Irwin 2004; Hulme 2009). In particular, human population size and various indicators of socio-economic behaviour have been shown to be linked to patterns of invasion (McKinney 2001; Pyšek, Kucera, & Jarošik 2002; Taylor & Irwin 2004; Hulme 2009), because they are surrogates of propagule pressure and changed disturbance regimes of natural systems (McKinney 2001; Lockwood, Cassey, & Blackburn 2005; Colautti, Grigorovich, & MacIsaac 2006; Krivánek, Pyšek, & Jarošik 2006). Biological invasions are often characterized by time lags between first introduction of a species in a new territory and the start of invasive spread (Kowarik 1995; Richardson et al. 2000). The duration of such time lag phenomena largely depends on species traits (e.g. life cycle), introduction history (e.g. amount of propagules or individuals introduced) and matching of the alien species autecology with the habitat conditions in the new territory (Pyšek & Richardson 2007; Bucharova & van Kleunen 2009; Pyšek, Krivánek, & Jarošik 2009) or the time necessary for phenological or genetic adaptations to new abiotic and biotic environments. For long-lived species like shrubs or trees such time lags may be substantial, reaching decades or centuries (Kowarik 1992, 1995; Pyšek et al. 2009, but see Daehler 2009). Invasion patterns in Europe are inconsistent across habitats (Chytrý et al. 2008). So far, temperate zonal forests are relatively little affected by invasions (Chytrý, Pyšek, Tichy, Knollova, & Danihelka 2005; Chytrý et al. 2008; Walter, Essl, Englisch, & Kiehn 2005). However, taking into account that forests are dominated by woody species with long generation periods and invasion into forests predominantly occurs when disturbances create gaps or clearings (Chytrý et al. 2005, 2008) leading to temporarily increased resource availability, which can foster plant invasions (Davis, Grime, & Thompson 2000), invasions in temperate forests may exhibit particularly long time lags. Hence, invasion resistance of temperate forests might have been overestimated. Based on the inventory of the alien vascular flora of 15 study sites in East Austrian lowland forest patches, we ask the following questions: (1) What is the diversity, structure and composition of the alien flora? (2) What are the temporal trends (time elapsed since the invasion of the species in Austria) in alien species composition and is there a change over time in the importance of pathways, regions of origin and life forms? (3) What are the effects of within-habitat diversity, occurrence of forest areas dominated by alien tree species and adjacent land use type diversity on the level of invasion? Data Study area The study area is situated in the Marchfeld region in the lowlands of Eastern Austria, 30 km east of Vienna (48◦ 17′ –48◦ 20′ N, 16◦ 35′ –16◦ 43′ E, Fig. 1). The flat area is part of the glacial Danube valley (165 m a.s.l.). The bedrock consists of gravel deposited by the Danube during the Ice Ages, which is covered by sandy alluvial soils. The climate is temperate pannonic, with cool winters and warm summers, average annual temperature being 10 ◦ C and annual precipitation 550–600 mm (1961–1990, Wiesbauer & Mazucco 1997). The Marchfeld is dominated by agriculture, so only few significant forests remain. However, in the central part of the Author's personal copy F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9 Fig. 1. Location of the study area in East Austria and of the 15 forest patches within the central Marchfeld region. ©BEV 2010 (T2010/70002). Marchfeld, several extensive forests of approximately 10 km2 exist in the environs of the villages of Deutsch Wagram, Straßhof and Gänserndorf. These forests have been replanted on formerly extensively grazed dry grassland in the 18th and 19th century (Wiesbauer & Mazucco 1997). Due to their poor growth, they are currently not or only extensively used silviculturally. Species data In the years 2008 and 2009, the alien vascular flora of 15 forest patches of different sizes (ranging from 2.4 ha to 47 ha) of the central Marchfeld was recorded (Fig. 1). The study sites are separated from each other by settlements and agricultural fields, and the minimum distance between individual study sites is 150 m. All sites are dominated by old-growth forest (stand age > 60 years), only small fractions of the sites are occupied by medium-aged forest (minimum stand age: 30 years), whereas young-growth areas are of negligible size. Due to their uniform abiotic characteristics (flat topography, identical soil types and climate) and history, these forest sites offer a unique possibility for a comparative assessment of invasions. For each forest patch, a set of variables was collected (see Table S1). Based on the digital AustrianMap (www.amap.at), we calculated the size of each site. Based on field data and aerial photos, we mapped and calculated the percentage of three different forest types to account for habitat diversity and forestry intensity within the sites: native broadleaved forests (for.nb) dominated by native oaks (Quercus cerris, Q. petraea, Q. robur), native conifer plantations (for.ncp) dominated by pine species native to eastern Austria (Pinus nigra, P. sylvestris), and forests dominated by alien broadleaved trees (Ailanthus altissima, Robinia pseudacacia; for.ab). To account for the influence of adjacent habitat diversity, the presence/absence of the following four habitats was recorded: large (two lane) roads (l.s.), railway (r.w.), crop fields (fi.), built-up areas (b.u.). We collated a complete list of alien vascular plants, the frequency was assessed in five frequency classes (see Table S2). 3 For analyses, we used presence/absence data only. We considered only alien species recorded after 1500 in Austria (neophytes), while old aliens (archaeophytes) were excluded (Richardson et al. 2000). To sample a species list as complete as possible, each site was visited at least four times in different months of the growing season to cover different phenological periods. To avoid size-related sampling biases, surveys in larger sites lasted longer, with total sampling effort being approximately 1 h per 4 ha. Data collection was done by systematically surveying all parts of the patches, while excluding forest edges. Taxonomically critical species were collected and verified or determined by specialists (see “Acknowledgements”). A few species could only be identified to genus level, but they were treated as species in the analyses. For each species, we collated a set of additional variables: first record in Austria (as a proxy for the local residence time), region of origin (continents), and dominant pathways (intentional introduction by horticulture [including ornamental trees] or forestry; unintentional introduction) based on the information provided in AliensAustria (2009). All species were assigned to life form categories: macro-phanerophytes (trees), nano-phanerophytes (shrubs), long-lived herbs (hemicryptophytes, geophytes) and short lived herbs (annuals and biennals). Based on the database AliensAustria (2009), each species was classified according to its habitat affiliation as forest species or as non-forest species. The latter mainly consist of ruderal and grassland species which occur in small gaps, along forest edges or tracks inside forests. Taxonomy and nomenclature follow Fischer, Adler, and Oswald (2008), for alien species not covered there, we followed the database AliensAustria (2009). Statistical analyses When testing for the influence of within-habitat and adjacent land type diversity, we used two approaches in order to account for the different sizes of the forest patches. First, log area was used as a covariate in the analysis. The independent variables are thus related to the residual variation of the diversity by size model. Second, all independent variables were standardized for the area effect by dividing through size. Two variables were used, which are assumed to determine the variation in species richness: Forest habitat diversity of the sites was expressed as the number of forest habitat types divided by area of the respective site (div.hab). To analyse the effects of adjacent habitat diversity, the presence and absence of adjacent habitats were summed and then divided by the size of the respective site (div.a). Since the number of alien species was the response, we used generalised linear models (GLM) from the Poisson family with a log-link function. Significance was tested by dropping the respective independent variable from the model with the size as a covariate using the likelihood ratio statistic and assuming a χ2 -test distribution under the null hypothesis. In addition, quasipois- Author's personal copy 4 F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9 Table 1. Numbers of alien species of 15 forest patches as explained by the ratio of forests areas dominated by alien vs. native tree species within each forest patch (for.r) using generalized linear models (GLM) with Poisson distribution and a log-link function (null deviance: 46.9, residual deviance: 17.2). The log of the size of the forest patch is used as a covariate to control for area-related species richness. Significance is determined by dropping the respective variable from the full model using the likelihood ratio statistic (LR) assuming a χ2 distribution. df, dev: the change in the degrees of freedom and the deviance respectively when dropping the respective variable from the full model; coeff: coefficient. log (size) for.r df dev coeff AIC LR p-Value 1 1 40.9 20.4 +0.25 −0.44 127.9 107.3 23.8 3.2 <0.001 0.073 son models were used accounting for overdispersion but as the results were the same, we here show the simpler Poisson models. Due to the limited number of investigated forest patches separate models were used for: (1) the ratio (for.r) between area dominated by alien (for.ab) and area dominated by native (for.ncb + for.nb) tree species within each forest patch. This ratio ranges between 0 and 0.7, because area of forests dominated by native tree species is higher than forest area dominated by alien trees in each site; (2) the diversity of habitats within (div.hab) and adjacent to (div.a) the respective forest patch. As a result of the limited sample size we accept the alternative hypotheses at an ␣-level of 0.1. We tested the effect of residence time of the alien species in Austria on their abundance on the local scale (number of invaded forest patches) using a Kruskal–Wallis–H test. To account for differences in invasion success of alien species of different habitat affiliations, we did this analysis separately for forest and non-forest species. We analysed temporal changes in the composition of the alien flora by testing for differences of the residence time in Austria for different pathways, regions of origin and life forms by means of KW-tests. Statistical analyses were carried out in SPSS Version 11.5.1 (Anonymous 2002) and R (R Development Core Team 2009). Results Diversity, structure and composition of the alien flora In total, we recorded 119 alien (neophyte) species in the 15 study sites (see Table S2). Six species were recorded in all sites, whereas 51 species were rare (recorded at most in two sites). Alien species numbers per site ranged from 25 to 69 species. Size of the forest patch has a large impact on alien species number (Table 1). The most important life form present are long-lived herbs (36% of all alien species), followed by shrubs (27%), short-lived herbs and trees (19% each). Tree species, however, colonized on average the highest number of sites (8.5 sites), followed by shrubs (5.7 sites), short-lived herbs (4.0 sites) and long-lived herbs (3.9 sites). North America, Europe and Asia are of nearly equal importance as regions of origin (29–32 species), and 14 species have originated in cultivation (anecophytes). No species native to other continents were found. Species introduced intentionally are by far the largest group (88% of all aliens). The pathway of overriding importance is horticulture (86% of all aliens), whereas only 2% of the species have been introduced for silviculture. Temporal change in composition of life forms, pathways and regions of origin Using the year of first record in Austria as a proxy for local residence time, invasion rate has been low until 1800. Since then, the number of aliens has accumulated much faster with a linear increase of 5 species per decade (Fig. 2A). Species introduced after 1900 into Austria make up 50% of all alien species recorded. Alien species of different life forms significantly differ in their residence time (p < 0.001, KW-test). Shrubs are the most recent invaders compared to aliens of other life forms (Figs. 2A and 3A). European and North American species have been introduced earlier into Austria, whereas species native to Asia or originated in cultivation are more recent introductions (Fig. 2B) (p = 0.016, KW-test). Importance of pathways has changed over time (Figs. 2C and 3B). Species introduced unintentionally are predominantly old introductions (86% introduced before 1900 into Austria), whereas only 46% of the species introduced for horticultural purposes have been recorded in Austria before 1900. Temporal differences between pathways are highly significant (p = 0.005, KW-test). Residence time of the species in Austria had a significant positive influence on the number of forest patches invaded (Fig. 4); this is true for forest and non-forest species (p < 0.001, KW-test), although for the latter the numbers of invaded forest patches are significantly lower at all time periods. Forest species were recorded throughout the forest sites, whereas non-forest species tended to invade gaps, forest tracks and young-growth areas. The effect of within and adjacent habitat diversity The GLM including size and ratio of alien vs. native forests explains 64% of the variance of the number of alien species. Size was clearly more important, but the ratio significantly contributes to the explained variance (p < 0.1; Table 1). A negative coefficient of −0.44 means that with every 10% more of alien forest area per site, approximately 1.6 alien plant species are added to the forest vegetation. Author's personal copy F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9 5 Fig. 2. Boxplots of the year of first record in Austria as a proxy for local residence time of alien species classified by (A) life form (ll = long-lived herbs, sh = shrubs, sl = short-lived herbs, tr = trees), (B) region of origin (AS = Asia, cult = originated in cultivation, EU = Europe, NA = North America), and (C) pathway (for = alien species introduced for forestry, hor = species introduced for horticultural reasons, unint = species introduced unintentionally). Both, within-site habitat diversity and adjacent land use diversity did not significantly explain the number of alien species in the second GLM. In addition, we tested the diversity variables in separate models and as additional independent variables in the above models, but the p-values always remained >0.1. Discussion Characteristics of the alien flora The number of alien species recorded in the 15 forest patches is exceptionally high, making up 11% of the total Fig. 3. Cumulative species numbers (using year of first record in Austria as a proxy for local residence time) for 15 forest patches (A). Cumulative species numbers for alien species of different life forms (A) and for alien species introduced by different pathways (B). Author's personal copy 6 F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9 ions (e.g. recent wide spread planting of ornamental shrubs) have contributed to this pattern, as both processes enhance invasion velocity at the landscape scale (Kowarik 2003b). Temporal changes in alien species composition Fig. 4. Number of forest patches (total n = 15) invaded by alien forest (grey bars) and non-forest (white bars) species classified by time of arrival in Austria (year of first record, 5 periods). alien flora of Austria (1110 alien vascular plants, Rabitsch & Essl, 2006) on a total area of 307.2 ha. In addition, this species number was recorded in forests only. Besides the proximity of extensive settlements and gardens leading to high propagule pressure, another reason for this high level of invasion may be attributed to the mild climate of the study region, as levels of invasion of aliens in Central Europe are higher in the lowlands (Pyšek et al. 2002; Chytrý et al. 2008). Although the Austrian alien flora is exceptionally well documented (Essl & Rabitsch 2002; AliensAustria 2009), several of the alien species recorded have not or only very rarely been found before in Austria (e.g. Amelanchier lamarckii, Anemone blanda, Fritillaria imperialis, Kolkwitzia amabilis, Ribes multiflorum) and testify the dynamics of alien species spread in Europe (Lambdon et al. 2008; DAISIE 2009). Furthermore, several species recorded only recently for the first time in Austria are wide-spread in our forest patches (exclusively shrubs as Cotoneaster divaricatus, C. dielsianus, Kolkwitzia amabilis, Ribes multiflorum, see Table S2). The incipient invasion of evergreen broadleaved shrubs and trees in the warmest parts of Central Europe due to global warming (Walther 2002; Walther et al. 2009) was confirmed, as several species, most of them being rare and evidently only recently escaped, were recorded (Euonymus fortunei, Viburnum rhytidophyllum). Escaped fruit and nut trees of adjacent gardens (e.g. Juglans regia, Malus domestica, Morus alba, Prunus spp., Pyrus communis) are conspicuously common. Interestingly, species of those life forms with the longest generation periods (trees, shrubs) occurred on average on more sites than species with shorter generation periods, although they are on average no longer (trees) or even for a shorter time recorded as alien in Austria. We argue that zoochory (many shrubs and trees belong to species dispersed by birds or mammals, e.g. Cotoneaster spp., Juglans regia, Prunus spp., Ribes spp.), and changes in horticultural fash- We used year of first record in Austria as a proxy for local residence time. This approach has been used widely in assessments of the influence of residence time on invasion success of alien species (e.g. Castro, Figueroa, Munoz-Schick, & Jaksic 2005; Pyšek & Jarošik 2005; Essl & Dirnböck 2008) and we consider it a robust measure of temporal patterns of invasion at the local scale, although it also bears its limitations (e.g. local invasion may in fact have started significantly later than on the national level). Using this proxy we infer that the temporal changes in composition of the alien flora are substantial with regard to regions of origin, life forms and pathways. In the last decades, shrubs, species native to Asia, originated in cultivation, or introduced by horticulture have increased disproportionally in the alien flora of the study sites. The overriding importance of ornamentals in the alien flora of the studied forest patches (86% compared to 57% of the national alien vascular plant flora, Essl & Rabitsch 2002) may be attributed to the combined effects of several factors. First, in Europe (Hulme et al. 2008; Lambdon et al. 2008) and Austria as well, horticulture is by far the most important pathway for alien vascular plants. Second, they mostly are and were introduced repeatedly into new regions, and horticultural usage of these species in different environments increases propagule pressure and the likelihood that these species will escape (Kowarik 2003b; Dehnen-Schmutz, Touza, Perrings, & Williamson 2007a, 2007b; Bucharova & van Kleunen 2009). All of our study sites are adjacent to settlements with gardens constructed in the 1950s to 1980s, where planting of ornamentals and the deposition of garden wastes in adjacent forests increase local levels of propagule pressure. Third, the numbers of cultivated ornamentals increased strongly in the last decades (Kowarik 2003a; Dehnen-Schmutz et al. 2007b). Predominance of early introductions in European and North American species compared to those originating from Asia or from horticulture indicates the importance of changes in the structure of international trade (Hulme et al. 2008; Hulme 2009). General and horticultural trade between Europe and temperate East Asia – the dominant region of origin for Asian origin – has belatedly gained momentum (Mitchell 2003; Hulme 2009). Species which have originated in cultivation mostly as results of intensive breeding efforts, have in several cases been created only during the last decades (e.g. Fallopia × bohemica, Galeobdolon argentatum). On the other hand, species native to adjacent regions of Europe or to the former European colonies in North America were introduced earlier, giving them a headstart. As expected and in concordance with recent studies on plant invasions in Europe (Pyšek et al. 2009; Williamson et al. Author's personal copy F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9 2009), residence time of the species in Austria had a significant effect on the number of invaded forest patches. Numbers of invaded forest patches increased stronger with residence time for forest species, and non-forest species invaded fewer forest patches, which reflects the fact, that the latter find only very localized appropriate habitats in small forest gaps or along tracks. The late date of first records of shrubs in Austria compared to other life forms was unexpected and may partly result from changing fashions in horticulture (e.g. recently increased plantation of ornamental shrub species of Cotoneaster spp. and Ribes spp.). Role of within-habitat and adjacent land use diversity As expected by biogeographic theory and empirical evidence (e.g. McKinney 2001; Pyšek et al. 2002; Essl & Dirnböck 2008), patch size explained a large part of the variance. Second to this variable, only the proportion of alien forests contributed to the number of alien species. That is, the presence and extent of forests dominated by Ailanthus altissima and Robinia pseudacacia contributed to higher numbers of alien species. Both species form dense stands, and particularly the latter one, which is also more widespread in the study sites, has severe impacts on ecosystem properties, especially by altering the nutrient cycle (Kowarik 2003b; Kowarik & Säumel 2007; Kleinbauer, Dullinger, Peterseil, & Essl 2009). A recent study for Austria has shown, that Robinia pseudacacia is currently invading a wide range of habitat types (including oak forests, the dominant habitat type in our study region) and that climate change will increase invasion risks substantially across all habitat types (Kleinbauer et al. 2009). Hence, both climate change and secondary releases by planting for forestry or landscaping (Kowarik 2003b) will contribute to future spread of Robinia pseudacacia in the forest patches. Our results show, that invasion of alien trees with strong impacts on ecosystem properties, facilitates invasion in Central European forests – a feature, which has been aptly termed “invasional meltdown” (Simberloff & Von Holle 1999). Surprisingly, within and adjacent land use did not influence the numbers of alien species. We argue that this is due to the fact that all sites were adjacent to gardens and built-up areas which led to high levels of local propagule pressure and which masked possible influences of other land use types. 7 invasions (Lambdon et al. 2008; Chytrý et al. 2008), and there are only few substantial and wide spread invasions (Chytrý et al. 2005; Walter et al. 2005). The cause of this paucity of invasions is primarily seen as a lack of available resources and strong interspecific competition (Davis et al. 2000), which limit recruitment and the establishment of alien species (Chytrý et al. 2008). However, as forests are dominated by long-lived woody species, time lag phenomena stemming from long intervals between disturbances (e.g. gaps created by windfalls or tree harvesting), and dispersal limitation (e.g. aggravated by isolation of forest patches) may be of particular relevance (Svenning & Skov 2004, 2007), as they may delay alien species spread and hence mask invasibility of forests. Additional data from transects laid through the study sites (Essl unpubl.) show that there are no significant correlations between stand age and numbers of alien species recorded. So, we conclude, that in forest succession numbers of alien species in the investigated forests does not decline significantly as forest succession proceeds (e.g. as a consequence of shading and lack of disturbance). Conclusions Our case study shows, that temperate forests are not resistant to invasions. High levels of propagule pressure, created by adjacent settlements and gardens, may lead to high and increasing levels of invasion, which ultimately may profoundly change the structure and composition of the vegetation. Further, invasion of alien species with strong impact on ecosystem properties (transformer species) may enhance invasion rates in Central European forests. Most tree species with strong impacts on native vegetation in Central Europe are strongly controlled by temperature, so climate warming will increase their potential distribution (Chytrý et al. 2005; Kleinbauer et al. 2009), and possibly the range of habitats colonized. Although the setting of our study sites is context specific (e.g. large adjacent settlements with gardens, lowland forests), observed low levels of invasion in Central European forests situated at a greater distance to settlements may turn out to be an ephemeral phenomenon. Spread of alien forest plants on the landscape level may ultimately, although possibly only over long time periods due to significant lag times, lead to increased levels of invasion. Resistance of temperate forests to invasion Acknowledgements Open habitats (e.g. industrial, man-made habitats, grasslands) are more receptive to invasions than closed habitats (i.e. forests; Chytrý et al. 2008). In Europe, temperate forests (except for forests shaped by natural disturbances, i.e. floodplain forests) are considered to be rather resistant to plant We are grateful to Christian Schröck and Oliver Stöhr for their assistance in determining critical specimens. We are grateful for constructive comments and suggestions of three anonymous reviewers and the managing editor K. Hövemeyer. Author's personal copy 8 F. Essl et al. / Basic and Applied Ecology 12 (2011) 1–9 Appendix A. 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